Conventional controllers in air conditioning systems may be of the general type shown in U.S. Pat. No. 3,809,314 entitled SELF-POWERED VARIABLE VOLUME AIR DAMPER CONTROL, U.S. Pat. No. 4,077,567 entitled PNEUMATIC TEMPERATURE RESET DIFFERENTIAL PRESSURE CONTROLLER, and U.S. Pat. No. 4,042,173 entitled METHOD AND APPARATUS FOR CONTROLLING VOLUME AIR FLOW.
Generally, controllers of the type under consideration are of the pneumatic type and employ duct or independent supply air in the control and operation of a pneumatic actuator which positions a flow regulating damper in response to the demands of the controller. A differential pressure across a fixed orifice is conventionally employed to provide a flow responsive signal representing the flow of conditioned air in the duct section associated with the damper. The differential pressure is applied across a diaphragm for operating a push rod or like output member in a pneumatic-mechanical motion transducer. The push rod is operatively associated with a pivotally supported lever which is also influenced by one or more reference or bias signals provided, for example, by adjustable minimum and maximum flow stops and by a signal responsive to a conditioned affecting a controlled space which receives the conditioned air. That is, the second signal may also be pneumatic and may comprise a temperature signal taken from a pneumatic thermostat in a controlled space which receives conditioned air from the duct and damper associated with the control system. When temperature control or temperature reset as it is commonly referred to is introduced, a pneumatic thermostat is usually employed with a pneumatic-mechanical motion transducer having a push rod or other output member acting in opposition to a bias or reset spring. The temperature or other air condition signal is combined with the aforesaid flow responsive and reference or bias signals to provide a composite signal acting on the aforesaid pivotally supported lever in opposition to the push rod or other output member of the flow or differential pressure transducer. The temperature or other air condition control or reset signal may be transmitted to the pivotally supported lever by a second pivotally supported lever.
The various signals acting on the first mentioned pivotally supported lever of the control system result in a composite signal at a control orifice which may comprise a flow responsive signal compared against a temperature reset signal. The control orifice establishes a pneumatic actuator operating pressure for movement and control of the damper in the conditioned air duct and for the maintainence of desired flow or volume conditions in the duct and for reset operation in accordance with the temperature or other air condition signal.
Control systems of the type described operate relatively efficiently over a somewhat limited operating range and are generally satisfactory except in those applications where an expanded operating range is required.
It is the general object of the present invention to provide an improved air conditioning control system of the general type employing a flow responsive signal and a second signal responsive to a condition affecting a controlled space receiving conditioned air, the system including a means for varying the area of the normally fixed flow orifice in the supply duct and for thus enhancing the effective operating range of the control system.